You can't just "make everyone upgrade", not without a time machine-- because there are transactions which may have been written arbitrarily far in the past, already signed, potentially lock-timed, whos signers (or at least their keys) have sailed off into the sunset.
If compatibility with their signatures is dropped those funds will be irreparably and irrecoverably destroyed.
So, for example, BCash deployed an earlier version of our schnorr signature spec (from before the taproot part was finished) in a "hardfork" but preventing destroying funds meant that they had to keep the ECDSA support around (duo to presigned transactions, hardware security modules, etc.) -- so they didn't escape any complexity in that change, they introduced a disruptive flag-day which introduced its own extra complexity.
> often hear a bunch of garbage that they can't make sense of
The changes are compatible so you know those extra fields are stuff "from the future" which you don't understand and know you can ignore.
> but they won't be able to make sense of it or accept
The recipient of funds always specifies their own rules, you'll never specify rules that you don't understand so there isn't any issue with not being able to accept it.
If compatibility with their signatures is dropped those funds will be irreparably and irrecoverably destroyed.
So, for example, BCash deployed an earlier version of our schnorr signature spec (from before the taproot part was finished) in a "hardfork" but preventing destroying funds meant that they had to keep the ECDSA support around (duo to presigned transactions, hardware security modules, etc.) -- so they didn't escape any complexity in that change, they introduced a disruptive flag-day which introduced its own extra complexity.
> often hear a bunch of garbage that they can't make sense of
The changes are compatible so you know those extra fields are stuff "from the future" which you don't understand and know you can ignore.
> but they won't be able to make sense of it or accept
The recipient of funds always specifies their own rules, you'll never specify rules that you don't understand so there isn't any issue with not being able to accept it.